dt calculation for sqe_fits now uses mean of diff instad of t[0]-t[1]

improves stability for non äquidistant sampled data 

SineTools.py

@@ -201,7 +201,7 @@ def seq_threeparsinefit(y, t, f0, periods=1):
             RuntimeWarning,
         )
     Tau = 1.0 / f0 *periods 
-    dt = t[1] - t[0]
+    dt = np.mean(np.diff(t))
     N = int(Tau // dt)  ## samples per section
     M = int(t.size // N)  ## number of sections or periods
 
@@ -303,7 +303,7 @@ def seq_fourparsinefit(y, t, f0, tol=1.0e-7, nmax=1000, periods=1, debug_plot=Fa
             RuntimeWarning,
         )
     Tau = 1.0 / f0 *periods
-    dt = t[1] - t[0]
+    dt = np.mean(np.diff(t))
     N = int(Tau // dt)  ## samples per section
     M = int(t.size // N)  ## number of sections or periods